Relatively large articles such as automobile parts and curbside refuse collection carts have been integrally molded with injection molding and rotational molding processes. Attempts are presently being made to make larger and larger molded plastic parts in the transportation industry. Plastic parts have the advantage of light weight, corrosion resistance and lower cost.
Thermosetting polyester filled with chopped fibers have been compression molded into relatively large sheets or panels. The surface finish is not particularly good. Decorative panels are typically painted.
Resin transfer molding (RTM) has been used to make some external body parts. A glass or graphite preform is positioned in a mold and a liquid thermosetting resin is injected into the mold. The thermosetting resin solidifies and forms the body of the part. These parts typically need a structural support and have a relatively poor surface finish. However, these parts have traditionally been painted because the surface finish has not been satisfactory enough for use without painting.
The resin transfer molded parts are not recyclable in that the thermosetting resins cannot be remelted and reused. Thus, reject parts must be scrapped and sent to the landfill. This scrapping of reject parts increases the ultimate cost of the acceptable parts. Even for those parts which are satisfactory, the parts must be sent to the landfill when the vehicle is scrapped or when they are damaged.
Vehicle manufacturers are designing more and more parts with ultimate disposal in mind. Thus, it is becoming more and more important to design automotive bodies with materials which can be recycled. According, thermosetting materials are not particularly desirable.
Thermoplastic resins with glass fibers have been extruded in sheet form. Glass fibers have also been used as a laminate in thermoplastic resin sheet form. The sheets are then compression molded to a particular shape. These parts are recyclable in that the thermoplastics can be comminuted and recycled. Compression molding has limitations with respect to certain shapes. For example, compression molded parts cannot be drawn very deeply and thus must be of relatively shallow configuration. Further, any holes in the sheet are required to be made with a secondary operation, thus adding cost to the finished product. Further, the parts are not particularly strong and require structural reinforcements to be used in a vehicle body, for example. Further, the surface finish is not particularly good.
Injection molding of thermoplastic resin has been used for many small articles. Larger articles require a larger clamping tonnage for the mold halves due to the pressure with which the thermoplastic resin is injected and forced to the limits of the mold cavities. Some large articles have been made but the parts themselves are not particularly structural. For example, curbside refuse carts in 96-gallon size have been injection molded in relatively large presses. However, these carts do not have close tolerance requirements. Further, fenders and doors have been made in an injection-molding process. The fenders and doors, however, are not load bearing and have little structural integrity. These panels must be attached to the frame of the car body. Further, the outer surfaces are always painted because of surface flaws where external surface finish is important. In one instance, a bumper fascia has been made by injection molding and not painted. The bumper fascia was not structural.
The injection molding of larger articles requires multiple drops (gates). Typically, all gates open simultaneously. The use of multiple gates typically produces multiple knit lines. When parts exceed five feet in any one dimension, the problem is exacerbated.
It has been proposed to make automotive bodies by molding portions of the frame and skin from plastic and joining the frame and skin together by bonding. These parts are generally dish-shaped and nest within each other. There are difficulties in forming the bonding surfaces without adding significant weight or without expensive mold designs.
Another problem with injection molding larger articles is that the size of the articles is limited by clamping tonnage. The larger the projected area of the article, the greater the clamping tonnage. Machines which have very large clamping tonnage are very expensive and difficult to house. Extremely large clamping tonnage injection molding machines are extremely rare because of high cost.
Still another problem with injection molding of large articles is distortion due to uneven densities of the thermoplastic material throughout the articles. When higher molding pressures are used, thermoplastic resin near a gate will tend to pack more densely than the resin near the ends of the mold cavity. As a result, a large injection-molded article will sometimes warp due to uneven density of packing of the thermoplastic material. In parts in which fit and finish are important, i.e., low dimensional tolerances, warping and packing is unacceptable. When material has high fiber content or filler, orientation can be a serious problem.
Klobucar et al. in U.S. Pat. No. 5,162,092 disclose a process for injection molding a thermoplastic backing or other synthetic resin to a carpet layer by suspending the carpet layer in a mold cavity, injecting thermoplastic resin into the mold cavity and injecting an inert fluid, such as nitrogen, into the mold at a relatively low pressure to assist in distributing the thermoplastic resin throughout all points in the mold.
According to the invention, an apparatus for molding a relatively large article comprises a pair of mold halves defining a mold cavity therebetween and at least first and second injection conduits, spaced from each other, in one of the mold halves for injecting molten thermoplastic resin into the mold cavity. Injection valves or gates are provided in the first and second injection conduits to control the flow of molten thermoplastic resin into the mold cavity. A controller is programmed to control the first and second injection valves to initially open the first valve and close the second valve during an initial time period in the injection cycle. The controller is programmed to open the second valve about the time when the molten thermoplastic resin arrives at the second injection conduit through the mold cavity from the first injection conduit. Further, a gas-injection conduit with a gas control valve is provided in one of the mold halves for injecting an inert gas into the mold cavity, preferably in a rib cavity. The controller is programmed to control the gas control valve to control the injection of inert gas into the mold cavity to force the molten thermoplastic resin to the edges of the mold cavity, preferably about the time the flow of molten thermoplastic resin to the mold cavity is discontinued. The controller is further programmed to discontinue the flow of molten thermoplastic material into the mold cavity through the first injection conduit at about the time the injection of molten thermoplastic material into the mold cavity through the second injection conduit is commenced.
The mold cavity preferably comprises an elongated rib cavity. The first and second injection conduits and the gas-injection conduit terminate in the rib cavity. The injection of gas into the rib cavity hollows out the rib and packs the thermoplastic resin at the sides of the rib cavity to enhance the strength and rigidity of a molded rib in the resulting article.
In a preferred embodiment of the invention, the one mold half has a third injection conduit spaced from the first and second injection conduits and has a third injection valve or gate to control the flow of thermoplastic resin therethrough. The controller is programmed to open the third injection valve or gate substantially simultaneously with the arrival of the molten thermoplastic resin from the first or second injection conduit at the third injection conduit and is programmed to close the second injection valve to discontinue the flow of molten thermoplastic resin to the second injection conduit.
In one embodiment of the invention, at least one of the first and second mold halves has pressure sensors to detect the pressure of thermoplastic resin at several locations in the mold cavity. The pressure sensors are operably connected to the controller to provide inputs to the controller as to the arrival of the molten thermoplastic resin at least at the second and third injection conduits.